Chess trainer
A training tool that helps players study by checking their moves against real engine analysis rather than a set of hand-written rules.
The decision I chose to validate against Stockfish rather than heuristics — objective truth over plausible-looking output. Move quality is not something a rulebook should be guessing at when a real engine can just answer the question.
Proof 236 automated tests (as of 2026-07-24). The app itself validates every move against a real Stockfish binary — that's the architectural decision. The suite that checks the app is contract-tested against a fake engine implementing the same interface, with expected values (golden fixtures) seeded from genuine Stockfish runs, plus one test that spins up the real engine end-to-end.
What it shows I build things that can be checked, not just demoed.